1. Field of the Invention
The present invention relates to a resin coated carrier used in an electrophotographic image forming apparatus for developing an electrostatic latent image formed on an image bearing member to visualize, a two-component developer including the resin coated carrier, a developing device using the two-component developer and an image forming apparatus.
2. Description of the Related Art
Image output devices employing an electrophotographic technology such as printers and copiers use, as a developer for developing an electrostatic latent image formed on an image bearing member to form a visible image, a two-component developer consisting of a toner and a carrier or a one-component developer consisting of a toner alone, for example. A magnetic brush development method that uses the two-component developer among them is excellent in image quality and high-speed printing compared to other development methods, and is therefore widely used.
An image forming apparatus using the magnetic brush development method is provided with a developer bearing member comprising, for example, a cylindrical-shaped metal sleeve and a magnet roller inside of which permanent magnets are provided as a magnetic field generating section with a N pole and a S pole arranged alternately. By causing the surface of the metal sleeve of the developer bearing member to carry the two-component developer and rotating only the metal sleeve with the magnet roller fixed, it is possible to transport the two-component developer to a development area that faces an image bearing member on which an electrostatic latent image has been formed. A development electric field applied between the developer bearing member and the image bearing member causes only a charged toner to electrostatically attach to the image bearing member to form a visible image.
The toner in the two-component developer is mixed and stirred with the carrier in a development unit including the developer bearing member so as to be charged through contact friction. In dry two-component development, an electrostatic force of the frictionally charged toner is used for electrically handling to form a visible image on the image bearing member, and it is therefore important to control a charge amount of the toner. Although the charge amount of the toner varies depending on various conditions in a system, it is desirable that a value of the charge amount of the toner is stabilized for stability of the system.
Furthermore, high-quality and high-speed printing tends to be considered as important in copiers and printers in recent years. In this case, charge stability of the developer becomes particularly important. To achieve formation of a high-quality image, it is necessary to arrange a determined amount of the toner to a determined place. In the electrophotographic method, handling of the toner is performed by an electrostatic force, and the toner is required to maintain high charge at a certain level or more so that the toner overpowers other external forces such as an adhesive force to be transported by an electric field. In addition, since the number of print sheets is increased with increase in speed of an image forming apparatus, it is strongly demanded to reduce the number of times or the labor of the maintenance and a developer that operates stably over a long term is desired.
For such a demand, the carrier needs to frictionally charge the toner to a desirable polarity and to a desirable charge amount over a long term constantly. The carrier generally used in the two-component developer for charging the toner is different from the toner and stays in the development unit for a long term, it is concerned that the charge applying capability is reduced due to toner spent or stress from stirring and mixing. In addition, there is also a concern that toner scattering resulting therefrom causes contamination in an image forming apparatus. Accordingly, a highly-durable carrier that withstands degradation over time and that is capable of stably maintaining the charge amount of the toner over a long term is required.
In order to meet such a demand for the carrier, a resin coated carrier having the carrier surface coated with a resin has been proposed. Specific examples thereof include a resin coated carrier having the carrier surface coated with a styrene-acrylic copolymer resin or a polyurethane resin each having high surface energy, and a resin coated carrier having the carrier surface coated with a fluorine resin having low surface energy. The styrene-acrylic copolymer resin and the polyurethane resin having high surface energy have great adhesiveness to a carrier core, but have a drawback that the toner is easily spent, whereas the fluorine resin having low surface energy has poor adhesiveness to a carrier core although being effective against the toner-spent, and therefore, there is a drawback that the resin coating layer separates from the carrier core in stirring in the development tank and stable charging is inhibited.
For the purpose of solving the drawbacks, many resin coated carriers having the carrier surface coated with a resin coating layer including a silicone resin have been proposed conventionally, however, even in the resin coated carrier including a silicone resin, the resin coating layer is scraped due to long-term use, thus a problem that charging performance of the carrier varies fails to be solved completely.
To solve such problems, for example, Japanese Examined Patent Publication JP-B2 7-72810 (1995) discloses an electrophotographic two-component developer in which a toner, a resin coated carrier and a carrier core satisfy 0.18≦T2/T1≦0.77 (wherein, T1 represents a friction charge amount of the carrier core before being coated with a resin and the toner, an absolute value of T1 is 15.1 to 30.2 μC/g, T2 represents a friction charge amount of the resin coated carrier and the toner, the friction charge amounts T1 and T2 have the same polarity, and the friction charge amount T2 is lower than the friction charge amount T1 by 3.5 μC/g or more in an absolute value) so that the charging performance can be stabilized even when the resin coating layer is scraped.
Further, Japanese Unexamined Patent Publication JP-A 9-288384 (1997) discloses a magnetic carrier in which a ratio of a saturated charge amount of a toner with a resin coated carrier to the saturated charge amount of the toner with only magnetic core particles falls within the range of 0.78 to 1.1.
With the advancement of a full-color image forming apparatus in recent years, a lot of improvements to a toner have been conducted, and a part of which is an improvement to toner external additives. The toner external additive has a function of providing fluidity for the toner and a function of an agent for assisting control for the charge amount of the toner. In a full-color image forming apparatus, an external additive having a large particle size, specifically having an average primary particle size of 50 nm or more, tends to be added for the purpose of improving toner transfer efficiency. However, when a developer composed of a toner to which the external additive having a large particle size added and a carrier, is used over a long term, the external additive having a large particle size is attached to the surface of the carrier and easily accumulated thereon so that normal friction charge of the toner and the carrier is prevented to reduce the charge applying capability of the carrier. Accordingly, in the case of using the toner to which the external additive having a large particle size added for a two-component developer, it is difficult to stably charge the toner over the long term.
With respect to such a problem, the electrophotographic two-component developer disclosed in JP-B2 7-72810 and the magnetic carrier disclosed in JP-A 9-288384 are not capable of suppressing the reduction of the charge applying capability of the carrier over the long term when being used with the toner to which the external additive having a large particle size added, thus reducing charging stability of the toner.